Method of alloying reactive components

ABSTRACT

Metal ingots for forming single-crystal shape-memory alloys (SMAs) may be fabricated with high reliability and control by alloying thin layers of material together. In this improved method, a reactive layer (e.g., aluminum) is provided in thin flat layers between layers of other materials (e.g., copper and layers of nickel). When the stacked layers are vacuum heated in a crucible to the melting temperature of the reactive layer, it becomes reactive and chemically bonds to the other layers, and may form eutectics that, as the temperature is further increased, melt homogeneously and congruently at temperatures below the melting temperatures of copper and nickel. Oxidation and evaporation are greatly reduced compared to other methods of alloying, and loss of material from turbulence is minimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/736,671 filed Jan. 8, 2013 and titled METHOD OF ALLOYING REACTIVECOMPONENTS, which is a divisional of U.S. Pat. No. 8,349,099 issued Jan.8, 2013 and entitled METHOD OF ALLOYING REACTIVE COMPONENTS, thedisclosures of which are incorporated herein by reference in theirentirety as if completely set forth herein below.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

This invention was made with the support of the United States governmentunder DARPA Contract number W31P4Q-05-C-0158.

FIELD OF THE INVENTION

The invention is directed to an improved method of fabricating alloyscomposed of elements (or other alloys) that have very different meltingtemperatures and whose components strongly react chemically with eachother, especially alloys whose proper function requires strictstoichiometry. The invention is more particularly directed to animproved method of fabricating shape memory alloys.

BACKGROUND OF THE INVENTION

The making of alloys is an ancient art. Bronze is stronger than copperalone as a result of alloying copper with zinc, tin, and other elements.Steel is a significant improvement over iron alone, and exhibits greaterstrength and toughness. Alloys are developed for specific requirementssuch as resistance to corrosion. Certain alloys exhibiting shape memoryproperties and superelasticity are used in aerospace, consumer products,and medical devices.

Shape memory alloys (SMAs) are intermetallic compounds that undergo anenergetic phase change such that the mechanical properties are verydifferent at temperatures above and below the phase transformationtemperature. The most common SMAs in practical use are Ti-Ni alloys,Cu-based alloys and Fe-based alloys. It has been acknowledged that thereare problems with the fabrication of commercial SMAs. In particular,shifting the content of a component, such as Nickel, can result in achange in the martensite start temperature or the transformation finishtemperature. See, Shape Memory Materials, J. Otsuka and C. M. Wayman(Eds.), Oxford University Press (1999).

The higher temperature phase, generally referred to as austenite, has asimpler crystal structure and is more rigid than the low temperaturephase, called martensite. A body that is deformed (stretched or bent)while in the low temperature phase will recover its original shape whenheated to above its transformation temperature, giving rise to a shapememory. In shape memory alloys, the phase transformation from austeniteto martensite can be induced by stress. When the stress is removed, thebody reverts to its original austenite and consequently recovers itsshape. This phenomenon is called superelasticity.

The strain recovery is much greater in shape memory alloys than inordinary metals. Single crystal SMA may recover as much as thetheoretical maximum. For single crystal CuAlNi this is nearly 10%strain, which can be described as ‘hyperelastic.’ Thus, while shapememory alloys transform from one solid crystal structure to another, andare capable of energy storage at greater densities than elasticmaterials, in hyperelastic transformations, the energy is absorbed andreleased at nearly constant force, so that constant acceleration isattainable. See also, U.S. Patent Publ. 2006/0118210 to Johnson forPortable Energy Storage Devices and Methods.

Useful devices are produced by pulling single crystals of CuAlNi frommelt by a method due to Vasily Stepanov. Successful pulling of singlecrystals requires very pure alloys with very strict composition control.

Ingots for crystal pulling are conventionally made by mixing copper,aluminum, and nickel pellets and heating in a furnace. Conventionally,similarly sized pellets of material comprising the alloy are weighed tothe fraction that the material represents in the alloy. The pellets arethen mixed together in a crucible and heated until the pellets melt andgo into solution. The components then engage in congruent melting. FIG.1 illustrates a CuAlNi ingot 100 cast using conventional methods. Thismethod has several drawbacks. Since the melting temperatures of theelements are disparate, the individual elements comprising the alloy donot readily mix. As the aluminum is melted in conventional methods (ca650° C.), it becomes reactive and reacts explosively and exothermicallywith the copper and nickel. This causes spattering, especially in smallingots, and may result in significant loss of mass during themanufacturing process. Additionally, pellets have a large surface tovolume ratio and have oxide surfaces that generate slag. Unless themixture is stirred mechanically, the components can segregate intolayers, as illustrated in FIG. 1. Even if stirred vigorously whilemelted, it may separate during cooling unless cooling is rapid. If alarge amount of alloy (more than a kilogram for example) is melted at atime, some segregation is almost certain to occur because cooling cannotbe rapid. While the uneven qualities of the ingot of FIG. 1 are anextreme example for an ingot manufactured using conventional methods,significant segregation of components is not unusual. Segregation leadsto variation in composition throughout the ingot that causes difficultyin pulling single crystals. Up to 80% of a batch of ingots has beenfound to be unusable for pulling single crystal shape memory alloy. Evenwhen single crystals are successfully pulled, the results are notreproducible. The transition temperature of the phase transformationthat gives CuAlNi its desirable shape memory properties dependscrucially on composition (to 0.1%).

A method of overcoming these difficulties is important to the commercialdevelopment of hyperelastic alloys. The invention described herein is amethod whereby small ingots of CuAlNi can be made with reproduciblecomposition and good crystal growth characteristics.

SUMMARY OF THE INVENTION

The methods of forming alloys described herein takes advantage of theobservation that some metals (in particular Al) react with other metals(e.g., Ni and Cu) when heated above their melting temperature (e.g., themelting temperature of Al). Therefore, it is advantageous to arrange thematerial in thin flat layers (providing large areas of contact betweenthe layers) so that the reaction can take place smoothly and completelyleaving less opportunity for undesirable events to take place. This isachieved by stacking 1-mm-thick layers of alternating metals (such as Cuand Al), and alternating these layers with alternating layers of othermetals (e.g., Ni with Al), so that the separate layers have large areasof contact. This stacking method may facilitate diffusion of the first,reactive, metal into the other metal(s), allowing the energy of thereaction to elevate the temperature of the mixture over a much largersurface of contact, avoiding inhomogeneities arising from theexplosive-type events that otherwise occur. For example, the formationof alloys of CuAlNi is of particular interest herein, and has beendeveloped with great success. Stacking alternating layers of the morereactive metal such as Al, with other metals (such as Cu and Ni) in thinlayers before heating may allow and even enhance diffusion of thereactive metal (e.g., Al) into the other metals (e.g., into the Cu andinto Ni), and the energy of the reaction may further elevate thetemperature of the mixture over the entire surface of contact, avoidingthe violent explosive-type events that may occur if difference intemperatures occur in nearby regions.

Although the methods described herein are directed mostly to alloyingmaterials by forming layers of elemental metals (e.g., Ni, Al, Cu), itshould be recognized that the methods may also be used with otherelemental metals and with alloys of metals. An alloy may be formed asdescribed herein by alternating layers of elemental metals and layers ofalloys, or by alternating layers of alloys.

This invention also provides for devices and apparatuses having at leastone component made of a single crystal shape memory alloy (SMA) formedfrom an ingot made according to the method of the invention. Singlecrystal SMAs are hyperelastic SMA that have properties enablingcomponents made from them to undergo large recoverable distortions. Suchdistortions can be at least an order of magnitude greater than thatwhich could be obtained if the component were made of non-SMA metals andalloys, and nearly an order of magnitude greater than can be obtainedwith polycrystalline SMA materials. Devices and apparatus havingcomponents comprised of hyperelastic SMA can serve as: actuators for theactive deployment of structures such as booms, antennae and solarpanels; actuators for releasing door locks, moving mirrors and fuelinjectors; flexures; constant force springs; connectors; dampeners;valves; microchip substrates; support members; non-explosive separationdevices; catheter guide wires; laparoscopic instruments; medicalimplants such as stents; micro-connectors; switches; circuit breakers;electronic test equipment including probe tips; flexible electriccables; heat conductors; consumer products such as safety valves,eyeglass frames and cellular telephone antennae; and many other devicesand apparatus in which large recoverable distortions of a component orcomponents can be advantageous.

One aspect of the invention is directed to a method of alloyingdissimilar materials. The method comprises the steps of: obtaining aplurality of dissimilar materials configured to provide large areas ofcontact between the plurality of materials; layering the dissimilarmaterials in an alternating pattern; heating the dissimilar materials;melting the dissimilar materials; mixing the melted dissimilarmaterials; and cooling the mixed dissimilar materials to form an ingot.Examples of dissimilar materials include materials selected from thegroup consisting of: aluminum, copper, nickel, manganese, zinc, andtitanium, nickel alloys, and copper alloys. Of course, other materials(both elemental materials and alloys) may be used. As is known to thoseof skill in the art, alloys may be formed by combining two or moreelements in a ratio that is not the desired final ratio, then dilutingthis combination by adding other ingredients. Thus alloys containing anyof the elemental materials described above (e.g., nickel, aluminum,copper, manganese, zinc, titanium, etc.) may be used as well as theelemental material or other alloys including the same elementalmaterials. Even though alloys may include the same elemental materials,they may still be considered dissimilar materials if the makeup (ratio,and/or concentration) of materials is different.

In one embodiment of the invention, the method also includes the step ofreacting the dissimilar materials before and during melting. In stillanother embodiment, the plurality of dissimilar materials includes oneor more sheets of aluminum and further wherein one or more sheets ofaluminum begins to melt prior to a melting of any other sheets ofmaterials. Where one or more sheets of aluminum are used, these sheetscan be layered with one or more layers of copper and nickel. An aspectof the invention also includes nesting a material within an adjacentmaterial. During the heating step, the materials used as startingmaterials do not erupt, or only nominally erupt. The arrangement of thematerials into layers (e.g., the order of the layers) may be based onone or more properties of the materials, such as reactivity, meltingtemperature, etc.

In another aspect of the invention, a method of alloying metals having aphase form dependent upon temperature is provided. The method mayinclude the steps of: obtaining a plurality of dissimilar materialsconfigured to provide large areas of contact between the plurality ofmaterials; layering the dissimilar materials in an alternating pattern;heating the dissimilar materials; melting the dissimilar materials;mixing the melted dissimilar materials; and cooling the mixed dissimilarmaterials to form an ingot. The dissimilar materials may includematerials selected from the group consisting of: aluminum, copper,nickel, manganese, zinc, and titanium. In one embodiment of theinvention, the method also includes the step of reacting the dissimilarmaterials before melting. In still another embodiment, the plurality ofdissimilar materials includes one or more sheets of aluminum, whereinthe one or more sheets of aluminum begins to melt prior to a melting ofany other sheets of materials. Where one or more sheets of aluminum areused, these sheets can be layered with one or more layers of copper andnickel. One aspect of the invention also includes nesting the materialwithin an adjacent material. During the heating step, the materials usedas starting materials do not erupt, or only nominally erupt.

Still another aspect of the invention is directed to a method of makingan ingot where dissimilar materials do not segregate while cooling, oronly nominally segregate. Ingots manufactured according to the method ofthe invention ideally have a uniform composition of greater than 50%,more preferably greater than 80%, even more preferably greater than 90%,and ideally at least 99% uniform. Ingots manufactured according to theinvention have a weight greater than 95% of the weight of the startingmaterials, greater than 50%, more preferably greater than 80%, even morepreferably greater than 90%, and typically greater than 99.9%.

In yet another aspect of the invention, a variety of devicesmanufactured from shape memory alloys is contemplated. The devicescomprise a mechanical component, wherein the mechanical component isformed of a hyperelastic material manufactured from an alloyed ingotformed from obtaining a plurality dissimilar materials configured toprovide large areas of contact between the plurality of materials;heating the sheets of material; melting the sheets of material; mixingthe melted sheets of material; and cooling the mixed metals to form aningot.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference in their entirety to the sameextent as if each individual publication or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 is a picture of an ingot of shape memory alloy made according toconventional methods wherein there is segregation of components;

FIG. 2 is a phase diagram for copper-aluminum-nickel with a fixed 3%nickel;

FIG. 3 is a diagram depicting the layering of materials for formingingots according to the method of the invention;

FIG. 4 is a flow chart showing the steps of the method for making analloy ingot according to a method of the invention;

FIG. 5 is a picture of an ingot of shape memory alloy made according tothe method described herein.

DETAILED DESCRIPTION OF THE INVENTION

Described herein are methods for forming single-crystal shape-memoryalloys (SMAs) with high reliability and control by alloying thin filmsof material together. In particular, described herein are methods offorming CuAlNi SMAs by first producing high-quality seeds (ingots) ofcopper, aluminum, and nickel to produce for pulling single crystal shapememory alloys, in particular superelastic or hyperelastic alloys. Thesemethods may also applicable to a wide range of alloys in which one ormore of the components are reactive. The method is an improvement upontraditional methods such as mixing and melting pellets. In this improvedmethod, a reactive layer (e.g., aluminum) is provided in thin flatlayers between layers of other materials (e.g., copper and layers ofnickel). When the stacked layers are vacuum heated in a crucible to themelting temperature of the reactive layer, it becomes reactive andchemically bonds to the other layers, and may form eutectics that, asthe temperature is further increased, melt homogeneously and congruentlyat temperatures below the melting temperatures of copper and nickel.Oxidation and evaporation are greatly reduced compared to other methodsof alloying, and loss of material from turbulence is minimized

FIG. 2 illustrates a phase diagram for copper-aluminum-nickel with afixed 3% nickel. This phase diagram illustrates the phase changes thatoccur when an ingot is made according to the methods of the invention.After heating to melting temperature the three elemental components aredissolved in each other. As the temperature decreases, saturation occursand one or more of the elements start to precipitate out of solution. Ifcooling is very slow, the materials can even segregate into layers. Forsuccess in pulling single crystals, the melt must be homogeneous. Aboule pulled from melt by the Stepanov method, which is known by personsof skill in the art, undergoes some degree of precipitation becausecooling after crystallization is gradual. In order to obtain an alloywith optimal properties, the material, after being pulled, is heated to950° C. and then quenched. Single crystal shape memory alloy materialsformed in this manner take a remarkable amount of deformation withoutfracture, and return to precisely their original shape in thehigh-temperature phase.

Method of Fabricating an Ingot of Alloyed Material

Described herein is a method of fabricating an ingot of alloyed materialwhich overcomes the yield and composition issues experienced when makingingots according to current methods. The method enables alloyingdissimilar materials to form an ingot wherein greater than 50% of theingots manufactured can be used to pull a single crystal SMA (asdiscussed below). More particularly, the method enables alloyingdissimilar materials to form an ingot wherein greater than 90%, morepreferably greater than 95%, of the ingots manufactured can be used topull a single crystal SMA. Further, the yield of material formed duringthis process is also greater than 90%, more preferably greater than 95%.In making an ingot a plurality of sheets of two or more materials isobtained. Persons of skill in the art of manufacturing ingot alloys willbe familiar with suitable materials, which include, but are not limitedto, aluminum, copper, nickel, manganese, zinc, and titanium. Sheets ofsuitably sized materials 310, 310′, 310″ are layered, such asillustrated in FIG. 3. Suitably sized materials are, for example,approximately 2 inches by 2 inches square with a thickness that is muchless than the length and width, but which may vary from material tomaterial depending on the desired percent-composition of any particularmaterial in the ingot. The layers may be any appropriate thickness, andthe different layers may have different thicknesses.

The layers can be further configured to facilitate nesting of one ormore layers within adjacent layers. Typically, the sheets will belayered in an alternating pattern so that, for example, two sheets ofaluminum are not layered adjacent each other, and each layer of copperor nickel is sandwiched between two layers of aluminum. The layeredsheets are placed in a crucible, such as a graphite crucible, and thenput into an oven. Depending upon the starting materials used for thealloy, the oven can be a vacuum or inert gas environment in order tocompensate for the fact that some materials, such as titanium, are veryreactive to oxygen when molten.

The sheets of material are then heated so that they melt. For example,the material is gradually heated to more than 1100° C. As aluminumbegins to melt at about 660° C. it reacts with the copper and nickelwith which it is in intimate contact and a eutectic is formed with amelting temperature of about 1000° C. To ensure homogeneity, the melt isvigorously stirred. Once the material is cooled in flowing argon, ithardens into an ingot which can be used as described in more detailbelow.

This method of making an ingot of alloy produces an ingot where thecomponent materials do not, or only nominally, segregate while cooling.Alloy ingots produced according to this method also have a uniformcomposition of greater than 99%, with very small precipitates thatdissolve upon heating. Additionally, during the process of making thealloy ingot, the sheets of materials tend not to erupt during heating,unlike ingots formed from pellets, so very little material is lostduring the casting process: ingot alloys produced according to thesemethods achieve greater than 95%, and typically greater than 99.5%, ofthe weight of the starting materials. Thus, where 1000 g of materials(e.g., Al, Cu, Ni) are used to make an ingot, the final ingot weighsmore than 950 g, and more often greater than 995 g.

Thus, in its simplest incarnation, as shown in FIG. 4, sheets ofdifferent metals 400 are layered adjacent one another 410, thecomposition is heated 420 and an ingot alloy is formed 430.

In a specific example, an ingot having a composition of CuAlNi suitablefor pulling single crystals (described below) is 81.4 weight percent Cu,14.1 weight percent Al, and 4.5 weight percent Ni. This compositionproduces crystals having a phase transformation with an Austenite finishtemperature of approximately −10° C. To make a one kg ingot of alloyedmaterial, 814 gm Cu, 141 gm Al, and 45 gm Ni are prepared in the form ofsquares 2 inches on a side and 1 mm thick. As illustrated in FIG. 3,these squares 310, 310′, 310″ are stacked in a suitably sized graphitecrucible (not shown), alternating Al with Cu and Ni. The crucible isplaced in a vacuum chamber, evacuated to less than 5×104 Torr, andheated in about 5 minutes to 1100° C. The mixture is stirred with agraphite paddle, and then allowed to cool to room temperature in about60 minutes or less. As illustrated, the squares of material can be sizedsuch that the squares are approximately the same length, width, andthickness; alternatively the material can be sized such that alternatingsquares are larger in length, width and/or thickness than an adjoiningsquare of material (as illustrated).

Turning back to FIG. 1, FIG. 1 illustrates an ingot manufacturedaccording to methods currently used in the art. In contrast, FIG. 5illustrates an ingot manufactured according to the methods of theinvention. As is evident from the pictures, the ingot of FIG. 1 has arough, blotchy non-uniform appearance, while the ingot of FIG. 5 has asmooth, uniform appearance.

Method of Fabricating Single Crystal SMA

Since single crystals cannot be processed by conventional hot or coldmechanical formation without breaking single crystallinity, a specialprocedure is required for shaping single crystals in the process ofgrowth as the crystal is pulled from melt, resulting in finished shapeas described in WO2005/108635. In order to consistently obtain a singlecrystal, the ingot from which the single crystal is pulled must beuniformly high in quality. When pulling crystals using ingots madeaccording to current techniques, only 1-2 of 5 ingots (i.e., less than40%) are of sufficient quality that the ingots can be used to pullsingle crystal SMAs. In contrast, using an ingot made according to thetechniques described herein results in producing more than 80% of usableingots. In most instances, more than 90% of the ingots are usable,greater than 95% of the ingots are usable, and most often up to 100% ofthe ingots are usable.

Single crystal SMA is made in a special crystal-pulling apparatus. Aseed of the desired alloy is lowered into a crucible containing a meltedingot of the alloy composition, and gradually raised up. Surface tensionpulls the melted metal along with the seed. The rising column cools asit leaves the surface of the melt and crystallizes a short distanceabove the surface of the melt. The rate of pulling is controlled tocorrespond with the rate of cooling so that a solid crystal is formed ata region that becomes a crystallization front. This front remainsstationary while the crystal, liquid below and solid above, travelsthrough it. The top surface of the melt can contain a die (of thedesired cross-sectional shape) that forms the shape of the crystal as itgrows. This procedure generally is known as the Stepanov method ofmaking single crystals.

From the Cu—Al—Ni phase diagram, rapid cooling (quenching) of the drawncrystal is necessary for production of single crystal beta phase thathas the desired hyperelastic properties. Starting with beta phase at850-1000° C., if the alloy is cooled slowly, the beta phase precipitatesas beta+gamma, and at lower temperatures, as alpha+gamma-2. Singlecrystal beta phase, which requires that Al remains in solution at roomtemperature, is formed by rapid cooling in salt water from 850° C. Atelevated temperatures, above 300° C., some decomposition graduallyoccurs; in fact, beta phase is not entirely stable at room temperaturesbut the time constant for decay is many years.

Devices Manufactured from Alloy Ingots of the Invention

A variety of devices, components of devices and improved devices can bemade using the materials described herein. For example, microactuators,miniature valves, electric switches, relays, optical switches, a varietyof medical devices, improved stents, stent covers, anastomosis devices,self-expanding stents or stent covers, to name a few. See, for example,the disclosures of U.S. Pat. No. 5,325,880 to Johnson et al. forShape-Memory Alloy Film Actuated Microvalve; U.S. Pat. No. 5,903,099 toJohnson et al. for Fabrication System, Method and Apparatus forMicroelectromechanical Devices; U.S. Pat. No. 5,960,812 to Johnson forFluid Flow Control Valve; U.S. Pat. No. 6,470,108 to Johnson for OpticalSwitching Device and Method; U.S. Pat. No. 6,533,905 to Johnson et al.for Method for Sputtering TiNi Shape-Memory Alloys; U.S. Pat. No.6,614,570 to Johnson et al. for Shutter for Fiber Optic Systems; U.S.Pat. No. 6,642,730 to Johnson et al. for Thin Film Shape Memory AlloyActuated Microrelay; U.S. Pat. No. 6,669,795 to Johnson et al. forMethods of Fabricating High Transition Temperature SMA, and SMAMaterials Made by the Methods; U.S. Pat. No. 6,729,599 to Johnson forLiquid Microvalve; U.S. Pat. No. 6,742,761 to Johnson et al. forMiniature Latching Valve; U.S. Pat. No. 6,746,890 to Gupta et al. forThree Dimensional Thin Film Devices and Methods of Fabrication; U.S.Patent Publications 2003/0170130 to Johnson for Micro-Dosing Pumps andValves; 2003/0002994 to Johnson et al. for Thin Film Shape Memory AlloyActuated Flow Controller; 2002/0195579 to Johnson for Liquid Microvalve;2001/0039449 to Johnson et al. for Thin-Film Shape Memory Alloy Deviceand Method; and 2002/0071167 to Johnson et al. for Shutter for FiberOptic Systems; and PCT Publication WO 2005/108635 of TiNi Alloy Companyfor Single Crystal Shape Memory Alloy Devices and Methods.

In its broadest concept, the present invention provides devices andapparatus having a component made of a single crystal SMA material whichhas the property of enabling a repeatable strain recovery of as much as24%.

Because the range of strain recovery is so far beyond the maximum strainrecovery of both conventional polycrystalline SMA materials and non-SMAmetals and alloys, such repeatable strain recovery property of singlecrystal SMA is referred to herein as hyperelastic. Further, materialsexhibiting hyperelastic properties are referred to herein ashyperelastic materials. Also as used herein, the phrase largerecoverable distortion means the magnitude of repeatable strain recoverydescribed above for a hyperelastic material.

Within the past two decades, SMA materials have become popular for useas actuators due to their ability to generate substantial stress duringshape recovery of large strains during temperature-induced phasetransformation. The energy density of such actuators is high compared toother alternatives, such as electromagnetic, electrostatic, bimetals,piezoelectric, and linear and volume thermal expansion effects ofordinary materials. The operating cycle of an SMA actuator includesdeformation during or after cooling, and subsequent heating whichresults in a temperature-induced phase transformation and recovery ofthe deformation. SMA actuation is favored where relatively large forceand small displacements are required in a device that is small in sizeand low in mass.

Shape memory is the ability of certain alloys to recover plasticdeformation, which is based on a diffusionless solid-solid latticedistortive structural phase transformation. The performance of shapememory alloy based actuators strongly depends on the amount ofrecoverable deformation. In turn, recoverable deformation itself is afunction of the lattice distortions which take place during martensiticphase transformation in the particular SMA. For an individual grain(single crystal) of SMA, the amount of possible recoverable strain afteruniaxial loading, depends on the particular crystallographic orientationof the deformation tensor relative to the crystallographic axes of thehigh temperature (austenite) phase and the sign of applied load (tensionor compression).

For a given deformation mode, the recoverable strain is stronglyorientation dependent, and for the various crystallographic directionsit differs by approximately a factor of two.

The recoverable deformation of these polycrystalline SMA alloys, due tothe lattice distortion during diffusionless solid-solid phasetransition, is substantially lower than is theoretically possible for agiven material. The main reason for this is that for a conglomerate ofrandomly oriented grains (as is normally the case for polycrystallinematerials), the average deformation will always be less than the maximumavailable value for a given grain. The diffusionless nature of phasetransitions in SMA results in strict lattice correspondence between thehigh temperature (austenite) and low temperature (martensite) lattices.As the symmetry of the martensite lattice is lower than that ofaustenite, maximum deformation in each grain can only be attained in oneparticular crystallographic direction. This means that for randomlyoriented grains (as normally is the case for polycrystalline materials),the average deformation will be at least a factor of two less than themaximum.

The restrictions imposed on a polycrystalline body by the deformationmechanism are another reason for diminished recoverable deformation inpolycrystals as compared with a single crystal. To maintain integrity ofthe polycrystal, deformation of each particular grain has to be lessthan that corresponding to the theoretical limit for lattice distortion.

Therefore, for polycrystalline material, resultant recovery is thevector sum of particular grain deformations over the whole range ofgrain orientations, and is significantly smaller than the maximum valuefor an individual single crystalline grain.

By comparison, recoverable deformation close to the theoretical value(lattice distortion) can be achieved in single crystalline SMA. Inaddition to the substantially increased recoverable deformation, absenceof grain boundaries results in increased strength and longer fatiguelife. Specifically, as a single crystal, the strength of the grain forCuAlNi SMA can be as high as 800 MPa with the potential limit forrecoverable deformation up to 9% and even higher for special deformationmodes. An additional advantage of a single crystal SMA is that not onlythe thermally induced phase transformation may contribute to therecoverable deformation, as in the case for polycrystals, but also thestress-induced martensite-to-martensite phase transitions. Depending onthe material, this additional contribution may be up to 15%, thereforethe total theoretical recovery can be as high as 24%.

The various device applications contemplated by the invention withhyperelastic single crystal SMA are constrained by the intrinsicproperties of the material and by its behavior during forming andmachining and other secondary manufacturing processes. For example, ithas been shown that exposure to high temperature and/or stress can leadto recrystallization and the formation of unwanted crystals. The knownforming and machining processes which are successful include lathemachining, electro-discharge machining (EDM), grinding, laser cutting,electro-polishing, and the like. These processes can be used tomanufacture many basic shapes of the hyperelastic SMA, including rods,ribbons, flexures, coil springs, leaf springs, serrated tubes, rubes,pins and bi-stable elements.

Single crystal shape memory materials have significantly smaller thermaland mechanical hysteresis than polycrystalline materials. This isadvantageous since less energy is absorbed in the material on eachcycle, less heating occurs and more of the energy is recovered duringthe shape recovery.

Single crystal SMA hyperelastic components of mechanical devicesgenerally provide a significant advantage over other device componentscurrently available because they enable large displacement at constantforce. For example, aerospace applications include actuators which maybe used as motors to gently deploy spacecraft components such as booms,antennae and solar panels. Other aerospace applications include usage asconstant force springs, flexures or connectors that need to accommodatevery severe deformation but which spring back once the constraint isremoved.

Commercial applications for hyperelastic SMA components are similarly ofwide scope. They may be employed as a significantly improved replacementactuator or flexure over prior art SMA actuator applications. Theseapplications include thermostatic valves, tools and instruments used inmedicine, and other applications such as eyeglass frames and cellulartelephone, antennae.

The invention contemplates devices and applications having hyperelasticSMA components, or including hyperelastic components, made fromsingle-crystal SMAs (or formed from seeds or alloys) formed by thealloying methods described herein.

Single-crystal SMAs formed by the methods described herein may be usedin Aerospace and Military applications: As an actuator for activedeployment of a host of devices including booms, antennae and solarpanels.

Single-crystal SMAs formed by the methods described herein may be usedas a flexure or constant force spring used for passive movement of coverdoors or hinges.

Single-crystal SMAs formed by the methods described herein may be usedas a connector where it is necessary to accommodate significant motionof adjacent parts. For example, heat pipes aboard spacecraft requiresuch connectors to carry heating/cooling capability across a hinge to adeployable.

Single-crystal SMAs formed by the methods described herein may be usedas a damper used to absorb or mitigate energy coming from nearbypyrotechnic release devices.

Single-crystal SMAs formed by the methods described herein may be usedas a valve for a broad range of temperatures including cryogenic. Suchvalves have applications aboard missiles and satellites that carrysophisticated instruments such as sensors or cameras that need to becryogenically cooled.

Single-crystal SMAs formed by the methods described herein may be usedas an actuator in arming and safing ordnance.

Single-crystal SMAs formed by the methods described herein may be usedas a substrate or support member for a surface or component which needsto accommodate large motion including applications on optical assemblieswhich require support and actuation (movement).

Single-crystal SMAs formed by the methods described herein may be usedas a non-explosive separation device of smaller size than such boltsthat are prior art.

Single-crystal SMAs formed by the methods described herein may be usedas a flexible heat conductor or heat sink.

Single-crystal SMAs formed by the methods described herein may be usedin medical devices. For example, they may be used for making catheterguidewires that are significantly more flexible than those currentlymade from stainless steel or polycrystal SMA. The CuAlNi alloy has nodetectable cytotoxicity effect on the human body, and thus is compatiblefor use in a non-implantable function such as a catheter; inlaparoscopic instruments where it is necessary to make tools which cantolerate large distortions; and/or in implants such as stents where thematerial can be made bio compatible by coating with Au.

Single-crystal SMAs formed by the methods described herein may be usedin automotive applications. For example, they may be used as an actuatorfor releasing door locks, moving minors and for driving fuel injectorvalves.

Single-crystal SMAs formed by the methods described herein may be usedin computers or electronics applications, for example, inmicro-connectors and switches where large displacement capability allowsfor more reliable assembly, and for the fabrication of smaller parts; inflexible cables for print-heads and the like; and/or as constant-forceprobe tips for electronic testing of wafers and microcircuits.

Single-crystal SMAs formed by the methods described herein may be usedin various commercial devices: as rings made for use as metallicconnectors to secure braid in cabling applications, in switches, relays,circuit breakers and electronic test equipment, etc.

Single-crystal SMAs formed by the methods described herein may be usedin various consumer products. For example, they may be used in safetyvalves, eye glass frames and/or automobile and cellular telephoneantennae.

Examples of Equipment with Hyperelastic Components

The present embodiment provides the use of hyperelastic SMA inapplications such as equipment for sports and other activities. CuAlNisingle crystal material stores an enormous amount of mechanical energywhen it is deformed, and then releases the energy when the deformingforce is removed. Unlike normally elastic materials, however the energyis stored and released at nearly constant force. These characteristicsmake this material desirable for use in equipment for use in a varietyof sports and other activities.

Examples of applications benefiting from SMAs fabricated by the methodsdescribed herein include: bicycle wheel spokes equipped with ahyperelastic part to eliminate transmission to the hands of shocks dueto small bumps in the road; running shoes and basketball shoes cancontain a hyperelastic cushion that will reduce fatigue and enable theplayer to jump higher; skis that have a degree of hyperelastic behaviorcan reduce the shock of bumpy or irregular snow conditions and therebyimprove control and provide a more comfortable, stable platform; as partof an ‘exoskeleton’ that enables a human to jump higher or survivedescending from a higher distance than normal.

The capacity for storage of mechanical energy is as much as 3 Joules pergram of CuAlNi and the majority of the energy is stored or released at aconstant force resulting in constant acceleration. A parachutist, forexample, wearing special boots containing a few hundred grams of CuAlNiwould be protected from injury resulting from hitting the ground at ahigher than usual speed.

Many of the above benefits will be most advantageous to amateurs,occasional athletes, and elderly people whose flexibility is impaired.

Snap-Through Hinge/Flexure Embodiments

The following embodiments provide devices such as hinges or flexuresmade of hyperelastic SMA that allow constrained relative motion withoutsliding or rotating components. These are used in space vehicles toprovide lightweight structures such as booms that must be folded forlaunch into space. Similar flexures can also be used to replace priorart eyewear hinges.

These embodiments incorporate single-crystal hyperelastic materials intodevices resembling tape-hinges resulting in superior load-carryingcapability.

For spacecraft applications, the hinges/flexures must bend through anarc of 180 degrees to be useful in folding structures such as booms thatare stored during launch in a minimal volume. Minimum size of the foldedstructure is achieved when the flexures bend through a minimal radius.In prior art implementations, flexures were made of thin steel curvedtape. Steel in thin tape form does not provide optimum rigidity andstrength for a functioning boom. This invention uses hyperelastic SMA inflexures capable of repeated recoverable large deformations to minimizesize, maximize strength, and provide good vibration dampingcharacteristics.

Among the design considerations for flexure design are that compressionrigidity and resistance to buckling of the flexures should be consistentwith that of the other components of the structure. These considerationsset specifications for the flexure: length, thickness, width, curvature.This leads in turn to a design for a sliding die-mold for making thehyperelastic components.

In this embodiment, a tape hinge or flexure is formed by making aportion of a thin-walled cylinder and fixing it to rigid members orstruts at the ends.

A principal feature of the invention is a “snap-through” action thatresists bending because of its cylindrical symmetry which is very rigidfor its mass, but when an applied force causes the flexure to buckle, itbends through a large angle with a smaller force. After buckling thereis little restoring force because of its shape, that is, bending througha severe bending angle at a small radius of bend is possible because ofthe hyperelastic quality of the flexure. The flexure returns to itsstraight cylindrical rigid shape with a snap action because rigidityincreases rapidly as the flexure assumes its cylindrical shell shape.

Performance of these devices, and their applicability, can be enhancedby increasing the recoverable strain, enlarging the stress tolerance,and extending the hyperelastic temperature range of the SMA materials.The method of deformation in tape-hinges results in non-uniform strain.

As the bending torque/moment is applied, the edge of the tape element isunder tension, resulting in strain. After buckling occurs, this strainremains, and a bending moment is applied such that the inner surface isunder compression and the outer surface is under tensile stress, with aneutral axis near the center of the cross-section.

Incorporating the SMA hyperelastic technology into a design in which allmechanical elements are in pure tension or pure compression, it becomespossible to build a structure that is very light, has a high packingfactor for stowage, has a minimum of moving parts, and is very rigid forits weight. It is also possible to make it highly damped againstvibrations. Hyperelastic alloys allow construction of structures thatare strong against buckling while attaining a sharp radius of bend forcompact folding.

It is desirable to make hinges that have no rotating or sliding parts.These devices can be used in spacecraft. One known form of hinge is acarpenter's tape hinge. Such a hinge may be made by bending an elongateelement having a thickness much smaller than the width and having acurved cross-section. Such an element has a ‘snap-action’. These hingeswhen made of steel or materials with ordinary elasticity are restrictedto a small thickness in order to control the degree of strain within theelastic limit of the material. Limiting the strain to elasticdeformation limits the rigidity that can be achieved with BeCu and steeltape-spring hinges. Thus such prior art hinges are limited to relativelylight loads, and structures incorporating such hinges are not as rigidas is desired.

A material having greatly increased elasticity will enable thefabrication of ‘carpenter's tape’ hinges with increased load-carryingcapacity. One such material is hyperelastic single-crystal copperaluminum nickel in accordance with the present invention. Thisembodiment provides a significant improvement in the performance of tapehinges by exploiting the properties of hyperelastic shape memory phasechange material.

While preferred embodiments of the present invention have been shown anddescribed herein, it will be obvious to those skilled in the art thatsuch embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. It should be understoodthat various alternatives to the embodiments of the invention describedherein may be employed in practicing the invention. It is intended thatthe following claims define the scope of the invention and that methodsand structures within the scope of these claims and their equivalents becovered thereby.

What is claimed is:
 1. A method of making a hyperelastic single-crystalCuAlNi shape memory alloy, the method comprising: forming an ingot ofCuAlNi by: layering a layer of aluminum adjacent to a layer of copperand a layer of nickel; heating the layers; melting the layers; mixingthe melted layers; and cooling the mixture to form the ingot; placing aseed of a desired composition for the shape memory alloy into a melt ofthe ingot; drawing the seed from the melt at a controlled rate so that asolid crystal is formed at a crystallization front; and rapidlyquenching the drawn crystal to produce a single crystal beta phase. 2.The method of claim 1, wherein the rapid quenching is in salt water from850° C.
 3. The method of claim 1, further comprising reacting the layersbefore melting.
 4. The method of claim 1, wherein the melting includesthe layer of aluminum beginning to melt prior to the melting of thelayers of copper and nickel.
 5. The method of claim 1, wherein the drawncrystal is heated to a beta phase temperature of 850-1000° C. and therapid quenching is from the beta phase temperature.
 6. A method ofmaking a hyperelastic single-crystal CuAlNi shape memory alloy, themethod comprising: forming an ingot of CuAlNi by: layering a pluralityof layers of aluminum with one or more layers of copper and one or morelayers of nickel, in an alternating pattern so that each layer of copperand each layer of nickel is sandwiched between two layers of aluminum;heating the layers; reacting the heated layers; melting the reactedlayers, wherein the plurality of aluminum layers begin to melt prior tothe melting of the one or more layers of copper and nickel; mixing themelted layers; and cooling the mixture to form the ingot; placing a seedof a desired composition for the shape memory alloy into a melt of theingot; drawing the seed from the melt at a controlled rate so that asolid crystal is formed at a crystallization front; heating the drawncrystal at a beta phase temperature of 850-1000° C.; and rapidlyquenching the drawn crystal from the beta phase temperature to produce asingle crystal beta phase shape memory alloy.
 7. The method of claim 6,wherein the rapid quenching is in salt water.
 8. A method of making ahyperelastic single-crystal shape memory alloy, the method comprising:forming an alloy melt by: layering dissimilar materials in analternating pattern to provide large areas of contact between thedissimilar materials; heating the layers of dissimilar materials;melting the layers of dissimilar materials; and mixing the melted layersto obtain homogeneity; placing a seed of a desired composition for theshape memory alloy into the alloy melt; drawing the seed from the alloymelt at a controlled rate so that a solid crystal is formed at acrystallization front.
 9. The method of claim 8, wherein the layering ofdissimilar materials comprises layering a layer of aluminum adjacent toa layer of copper and a layer of nickel whereby the alloy melt is CuAlNiand wherein the seed is CuAlNi.
 10. The method of claim 9, wherein themelting includes the layer of aluminum beginning to melt prior to themelting of the layers of copper and nickel.
 11. The method of claim 8,further comprising: reheating and rapidly quenching the drawn crystal toproduce a single crystal beta phase.
 12. The method of claim 11, whereinthe rapid quenching is in salt water.
 13. The method of claim 11,wherein reheating the drawn crystal is to a beta phase temperature of850-1000° C. and the rapid quenching is from the beta phase temperature.14. The method of claim 8, further comprising reacting the layers beforemelting.
 15. The method of claim 8, further comprising, after mixing,and prior to placing the seed: cooling the mixture to form an ingot; andheating the ingot to form another melt of the alloy into which the seedis placed.